In the final stages of integrated circuit manufacturing, packaging, and testing, the integrated circuit package leads or terminals are finished with solder. The package leads are immersed in solder which flushes and cleans the leads and covers the leads with a solder finish for good electrical contact.
According to the conventional method, the leads depending downwardly from the sides of the package are solder finished by immersion in a "wave" of solder which rises from below the package immersing and covering the leads. The wave is generated in a solder pot or solder reservoir with the integrated circuit packages suspended or conveyed above.
In the monorail system for conveying and solder finishing the packages, the integrated circuit packages are conveyed along a track riding on a pair of adjacent rods which form the "monorail". The row of leads along each side of the packages depend in a downward direction on each side of the monorail. The track is assembled from track sections and a chain conveyor with extending fingers or lugs pushes the packages along the monorail formed by the track sections. The monorail track sections convey the packages over the solder pot or molten solder reservoir where a standing wave of solder is formed. The IC package leads pass through the crest of the standing wave and are thereby immersed, flushed and cleaned, and solder finished.
The track section over the solder pot or molten solder reservoir is referred to as the "solder bridge" and provides the locus of solder finishing. The solder bridge is a monorail track section where the depending leads along the sides of the package on either side of the pair of suspending rods are immersed in the standing wave as they are pushed along the track by an extending finger or lug of the chain conveyor. Alternatively, as many as 200 or 300 IC packages are mounted and suspended in a "pallet" over the solder pot and a large standing wave immerses the leads from below.
A disadvantage of the conventional methods of solder standing wave immersion is that excess solder accumulates on the leads, bridging between leads or between leads and monorail track section support rods. Solder also tends to accumulate on the package track section and conveying equipment. For example, fingers or lugs extending from the chain drive tend to pick up solder from the solder pot wave or solder reservoir wave. Solder also tends to accumulate along the monorail suspension or support rods of the track sections requiring regular cleaning. Similarly, where pallets are used for mounting and suspending IC packages, the solder tends to accumulate on the pallet equipment.